268 Meteorological Observations at Hudson, Ohio. 
the phenomena of the vapor and the air, we perceive the depen- 
dence of each upon temperature. Columns thirteen and fourteen 
in the preceding table, exhibit the pressure of the vapor as de- 
duced from the observations of the hygrometer on page 271; and 
subtracting these numbers from the total pressure, we obtain the 
pressure of the gaseous atmosphere, shown in the last two col- 
umns of the table. 
We perceive then that the pressure of the vapor is greatest in 
the hottest months, and at the hottest hour of the day. On the 
contrary, the pressure of the gaseous atmosphere diminishes in 
the summer months, and augments in the winter months; it also 
diminishes as the heat of the day increases. 
The cause of this phenomenon is obvious. As the temperature 
of the day increases, the air in contact with the earth’s surface 
becomes warmed; it rises, and a portion diffuses itself in the 
higher regions of the atmosphere, over spaces where the tem- 
perature at the earth’s surface is less. 'Thus the weight of the 
column is diminished. As the temperature declines, the column 
contracts, and receives in turn a portion of air which passes over 
in the upper regions from spaces where a higher temperature 
prevails, and thus the pressure is increased. 
he advantage of thus separating the pressure of the vapor 
from the pressure of the gaseous atmosphere, is apparent from the 
fact that the total pressure has two daily maxima and minima, in the 
explanation of which, much ingenuity has hitherto been wasted. 
The whole mystery is now solved by the bi-hourly observations 
at the English meteorological observatories. 'The vapor pres- 
sure and that of the gaseous atmosphere have each but one daily 
maximum and minimum, as might be expected from the fact that 
there is but one daily maximum of temperature. But the mo- 
tions of the vapor and the gaseous atmosphere following different 
laws; and their maxima occurring at opposite hours of the day, 
the sum of their effects, or the total pressure as shown by. the 
barometer, exhibits two daily maxima and minima, which occur 
at different hours from the maximum and minimum of tem- 
perature. 
The following table exhibits the instances, corrected for capil- 
_ and temperature, in which the barometer has risen above 
